Using a shaker filled with ice, add the tequila, grapefruit juice and sugar syrup (if you're using it) Squeeze in the juice of the lime wedge. Shake until well combined.
Pour in to a tumbler filled with ice and top with soda.
Garnish with the spent lime wedge.
Notes
If you wish to rim the glasses with salt, use a lime wedge to wipe the rims then invert the glasses in a shallow dish of salt.
The sharpness of grapefruits vary a lot so you may need to add a little agave or sugar syrup prior to shaking. I find 10ml sugar syrup to be just right to balance out the drink a little.
Recipe by A Life of Geekery at http://www.alifeofgeekery.co.uk/2015/07/paloma-my-favourite-tequila-cocktail.html